The present invention relates to a method for producing a saponified ethylene-vinyl acetate copolymer (hereinafter abbreviated as xe2x80x9cEVOHxe2x80x9d).
In the production of EVOH, it is necessary to remove impurities remaining or adhering on chips (also may be referred to as pellets), since they cause discoloration or fish eyes at the time of molding into a film or the like. Furthermore, the solvent used in the production, such as methanol, needs to be recovered as much as possible from the viewpoint of economic efficiency and reducing pollution.
To remove impurities and methanol from chips, conventionally, a method in which chips are washed by bringing them into contact with water, and thereafter repeating the step of centrifugal dewatering, has been employed. However, this washing method requires a large amount of water and takes a long time.
JP 50(1975)-4188A proposes a method in which a solution containing EVOH chips is introduced into a washing column (tower-type apparatus) through an upper portion thereof, and a washing solution containing an acetic acid ester is introduced into the washing column through a lower portion thereof, and these solutions are brought into contact with each other while flowing in countercurrent to each other, thereby continuously washing the chips. This method is excellent in the efficiency of washing. However, the use of an acetic acid ester as a washing solution is associated with problems with respect to safety and cost.
Thus, it is desirable to provide a method for producing EVOH efficiently and safely, in which the washing process of EVOH chips is improved, and thus in which visual imperfections are inhibited when the EVOH is formed into a molded product such as a film.
The present invention provides a method for producing EVOH, which comprises washing EVOH chips, wherein the chips containing methanol are introduced (preferably continuously) into a washing column through an upper portion thereof; water is introduced (preferably continuously) into the washing column through a lower portion thereof; while the methanol concentration in the solution in the upper portion of the column is maintained in the range of 25 to 45 wt % and the chips are allowed to fall from the upper portion of the column, methanol and water are discharged (e.g. continuously) from the column through an upper portion thereof; and the chips are taken out of the column (e.g. continuously) through a lower portion thereof with water that contains methanol in a lower concentration than the methanol concentration in the solution in the upper portion of the column. According to this method, EVOH chips can be washed continuously and safely. When introducing chips including impurities with methanol into the column, at least a portion of the impurities can be removed with the solution discharged from the upper portion of the column.
In the above method, an adjusting solution including methanol may if necessary be introduced into the washing column through an upper portion thereof.
The assessment of the methanol concentration in the solution in the upper portion of the column should be conducted on solvent contained in the upper 20% of the column volume. Preferably, the assessment is made on solvent in the top 5%, more preferably the top 2% of the column volume. In these cases, an average concentration of methanol in the solvent is defined as the methanol concentration. Most preferably the assessment of methanol concentration is made by testing the solution discharged from the upper portion of the column.
Sodium acetate is one of the impurities which is particularly desirable to remove from the EVOH chips so as to inhibit visual imperfections of an EVOH molded product. Thus, in the above method, it is preferable that the EVOH chips are washed so that the content of sodium acetate in the chips taken out of the column is not more than 500 ppm, preferably 200 ppm (by weight throughout, unless otherwise stated).
Furthermore, it is preferable that the methanol concentration in the chips taken out of the column is not more than 10,000 ppm.